This invention relates to an analogue filter system and more particularly to a novel analogue filter system capable of effecting a continuous real time processing of an analogue signal without any phase distortion.
Fluctuation of the base line of an analogue signal such as a biological signal, especially a cardiogramic signal, has difficulties with the recording, display or processing of a cardiogram so that it is necessary to eliminate such fluctuation.
The fluctuation of the base line is caused mainly by the polarization voltage of the induction electrode so that elimination of the polarization voltage is the most efficient solution for stabilizing the base line. However, any effective solution has not yet been made.
In electric circuits, various countermeasures for stabilizing the base line have been proposed including a method of eliminating the fluctuating component by an analogue filter, and a method wherein the fluctuating component is detected by an analogue filter and the fluctuating component is subtracted from the original signal. However, when a signal is passed through an analogue filter, the phase of the signal is distorted greatly owing to the phase shift caused by the filter, especially due to a rapid phase variation near the cut off frequency, thus making it difficult to obtain a stable base line. Although a logical filter provided by an electronic computer has been used to eleminate the phase distortion, such a method is not only expensive but also requisite for a large number of multiplying operations for each point, hence a long operation time especially at low cut off frequencies, so that it is difficult to perform a real time processing with a miniature computer.
However, since, in many applications, the biological signals are processed by an electronic computer, the analogue filter is required to be able to continuously perform the desired real time processing of the analogue signal.